I hate to sound like an idiot, but can someone explain to me the benefit of a larger turbo. I recently installed a DD Jammer turbo (I believe an H1 on steroids.) However, my boost is lower than that of my stock HX35 and it spools up much slower too. What am I gaining?

Does 30lbs of boost from a smaller turbo not produce the same amount of air as 30lbs from a larger turbo? This is what is confusing me.

With all of the additions that I have recently done, my EGT's remained the same as my somewhat stock configuration of an EZ, RV275's, Scotty's II and 4" exhaust.

I thought that a larger turbo was supposed to produce MORE boost than a smaller turbo. I was told by DD that I should get no more than 36lbs of boost from this turbo. This is about what I got from my stock turbo.

I hope someone can see where I comming from and shed some light on this subject. I have tried searching for information on this but have come up empty handed.

 

Actually, the engine could care less about boost pressure. What it is interested in is the mass flow (lbs per minute) of air. If you're running a comp and big injectors, yet the EGT is the same as with an EZ and 275's, the bigger turbo is doing its job.

Think of it this way - your stock HX35 had (I'm assuming) a 12 sq cm turbine housing. 30 PSIG boost, pushing against the engine restriction (ports, valves, etc.) and eventually exhausting through a 12 sq cm turbine housing would produce a mass flow of X lbs per minute. Now, with the larger turbo, you have a larger turbine housing, right? Therefore, the flow restriction contributed by the turbine side of the turbocharger should be less. This means for the same 30 PSIG boost, your air mass flow might be X + 20% (or more) lbs per minute, thanks also to the bigger turbo compressor, even though the geometry of the engine ports and valves hasn't changed. That extra mass flow of air allows you to burn more fuel and make more BHP without increasing EGTs.

It's like pushing water at 30 PSIG through a 1/2" garden hose versus a 5" fire hose - which will move more water?

Does this help?

Rusty

On edit - Boost pressure does affect how much air (by mass) is trapped in a given cylinder and available for combustion. It, therefore, is not as totally inconsequential as I might have originally made it sound.

 

Well described, RustyJC!

 

I was also under the impression that a bigger turbo could provide more air at a lower turbo shaft rpm thus creating less intake charge heat.

 

The intake charge heat is proportionate to the rise in pressure not the speed of the turbocharger shaft itself. Though there are some factors for effieciency, but those temperature values aren't of significance. The big temp rise is from atmosphere to the manifold pressure.

 

The stock turbo should've been plenty for your setup. Need to upgrade turbo when you have too much fueling for the air you can supply. That's my understanding.

Eventually a turbo does "max out"..producing mostly heat.

 

With all respect, I'm maxed out on EGT's with an EZ and DD2's. I don't know the specifics of the "Jammer 2" injectors, but with those and a Comp, I really think he's way outside the map of an HX35 - he's probably 50-75 BHP over me, at least. I know I'm gonna have to do something with the turbo before I can add any more fuel.

Rusty

 

I see two configurations...one in the body of message and one in the sig. I was basing my response on the body (RV 275's and EZ).

I agree....jammers and a comp would be pushing it if there's any weight.

 

Sorry, My sig contains my current configuration. The body of the message was what I had before all of the upgrades. We are dynoing the truck this Friday. We tried last week, but the comp box was not working. That has since been fixed. However, I hit 353 RWHP and 701 lbs Torque without the comp box.

My problem is that I am hitting redline before my turbo hits 30 lbs. We made some adjustments and hopefully can hit the 38-40lbs I am supposed to get from it.

 

Well...you're beyond my limited experience. I'd check with DD since you have their stuff. Maybe outlet size to build boost quicker???

 

The above explantions are very good. But another way to look at it is take a quart jar and fill it with marbles consider this the small turbo at 30# boost. Now take another quart jar and fill it with BB's, this is the large turbo at 30# boost. Both the marbles and the BB's represent the oxygen molecules per volume of air. See the difference? Cool air means more oxygen in a given amount of air.

Another factor that comes into play is the ability to get all of the hot exhaust air out..... Drive pressures are just as important as boost pressure. As drive pressure goes above boost pressure (just guess what drive pressure has to be to support 400+ HP with an HX35 ) efficiency goes way down along with HP and your egt's go up.

Doug

 

if you were only making 30psi on an HX35, you should have kept the HX35 IMHO.

Forrest

 

I hope to clear this up for you,wrk24wheel.Bigger is not always better.You have to match the turbocharger to the engine. Your engine does not move enough exhaust air through the turbine to accelerate it fast enough at low speeds.Driving the bigger turbine at low speeds,is like driving your truck and taking of in third gear it won't drive well will it,if you keep your foot into it will eventually speed up,but it will take for ever to get there.The smaller turbochargher accelerates faster because it has less mass and the load of air is smaller.On the exhaust side you also need to match the exhaust housing so as to channel all the exhaust gasses to the turbine to drive it.To big a housing there is not enough velocity in the exhaust to spin the wheel. To small a housing the engine has to work to hard to get out the exhaust hence high EGTs.Hope this helps